Surface Reaction Processes Research Articles

Fundamental and technological aspects of glass studied by nuclear techniques

Abstract Nuclear techniques (Rutherford back-scattering and nuclear reactions) appear very useful in the near surface analysis of glass. These techniques offer a non-destructive method of analysis, with high depth resolution without radiation effects induced by the ion beam. In this paper we present some experimental results concerning the analysis of glass surfaces obtained by different treatments. Initially the water vapour attack kinetics was investigated with the 23Na(ρ, α)20Ne nuclear reaction. Measurements were performed on three different flat glasses obtained by different industrial processes. Another aspect of particular importance from both scientific and applied viewpoints is the kinetics of the surface reaction and diffusion processes occurring in silicate glasses. In particular the Na+⇄K+ interdiffusion has been investigated with the RBS technique at different temperatures in aluminosilicate and borosilicate glasses. Finally, a technological aspect of great interest is connected to the optica...

Microstructure and properties of rf-sputtered amorphous hydrogenated silicon films

The effect of both total gas pressure (Ar+H2) and partial pressure of H2 during the rf-sputtering of amorphous hydrogenated silicon films was investigated. It is shown that decreasing the total gas pressure (PT) leads to a number of consistent and desirable film characteristics including no post-deposition oxidation, essentially zero etch rate, no resolvable microstructure, predominant monohydride bonding, and low optical band gap. These lowest PT films contain ∼17 at. % H and 4 at. % Ar, have reasonable deposition rates (60–70 Å/min) and are in a high state of compressive stress. The relations between the deposition variables and the more fundamental surface reaction processes are discussed. Specifically, we demonstrate that Ar ion bombardment effects, due to high negative substrate self-biases, are dominant at low PT.

Recent progress in understanding environment assisted fatigue crack growth

Metal fatigue has been well recognized as an important cause for failure of engineering structures. In most applications, fatigue damage results from the conjoint actions of the cyclically applied stress and the external (chemical) environment, and is therefore time dependent. Understanding of this load-environment interaction is essential to the formulation of rational life prediction procedures and to the development of realistic materials evaluation and qualification tests. Research over the past 15 years has led to the suggestion that the rate of fatigue crack growth in an aggressive environment, (da/dN)e, is the sum of three components—the rate of fatigue crack growth in an inert environment, (da/dN)r, which represents the contribution of “pure” fatigue, a cycle-dependent component, (da/dN)cf, that requires the synergistic interaction of fatigue and environmental attack, and the contribution by sustained-load crack growth (i.e., stress corrosion cracking) at K levels above K Iscc, (da/dN)scc. Recent fracture mechanics and surface chemistry studies have provided a clearer understanding of the cycle-dependent term, and, hence, a more complete understanding of environment assisted fatigue crack growth. (da/dN)cf results from the reaction of the environment with fresh crack surfaces produced by fatigue, and is a function of the extent of reaction during one loading cycle. For highly reactive alloy-environment systems, this contribution depends also on the rate of transport of the aggressive environment to the crack tip. The experimental basis and the development of models for transport and surface reaction controlled fatigue crack growth are reviewed. Interpretation of the effects of partial pressure of the aggressive environment and cyclic load frequency in terms of surface reaction and transport processes is discussed. Implications in terms of service performance and life prediction procedures are considered.

The dissolution of calcite in CO 2-saturated solutions at 25°C and 1 atmosphere total pressure

The dissolution of Iceland spar in CO 2-saturated solutions at 25°C and 1 atm total pressure has been followed by measurement of pH as a function of time. Surface concentrations of reactant and product species have been calculated from bulk fluid data using mass transport theory and a model that accounts for homogeneous reactions in the bulk fluid. The surface concentrations are found to be close to bulk solution values. This indicates that calcite dissolution under the experimental conditions is controlled by the kinetics of surface reaction. The rate of calcite dissolution follows an empirical second order relation with respect to calcium and hydrogen ion from near the initial condition (pH 3.91) to approximately pH 5.9. Beyond pH 5.9 the rate of surface reaction is greatly reduced and higher reaction orders are observed. Calculations show that the rate of calcite dissolution in natural environments may be influenced by both transport and surface-reaction processes. In the absence of inhibition, relatively short times should be sufficient to establish equilibrium.

Influence of surface recombination in satellite atomic oxygen measurements inferred from the AEROS‐A mass spectrometer

Although the assumption of complete and immediate recombination of atmospheric atomic oxygen at the surfaces of the open ion source of the AEROS‐A mass spectrometer is reasonable below 300 km, application of this assumption at higher altitudes results in a substantial underestimation of the ambient density of atomic oxygen. A two layer surface model seems necessary to explain the observations. Parameters for several surface reaction processes are estimated.

An experimental study of electronic factors in the dehydrogenation of ethanol on thin film ZnO

Electron transfer mechanisms involved in the adsorption and catalytic dehydrogenation of ethanol on thin-film zinc oxide were investigated. A primary objective was to measure changes in the electrical conductivity of the catalyst resulting from adsorption and surface reaction processes, and to study the effects of electrical fields in activity, selectivity and adsorptive capacity of the zinc oxide. At low temperatures (<200 °C) adsorption-conductivity experiments indicated that ethanol adsorbs as an electron donor, and at higher temperatures where appreciable surface reaction occurred (>225 °C) the composite adsorbate layer, ethanol plus reaction products, also behaved as a donor. Individual components hydrogen, water, and ethylene adsorb as weak donors at 150 °C, while acetaldeyde adsorbs as a strong acceptor under such conditions. These results indicate that under reaction conditions the adsorbed layer consists primarily of ethanol and hydrogen product, the reaction step involving immediate desorption of acetaldehyde product. Field-effect experiments were conducted for temperatures from 100 to 250 °C. The activity and selectivity of the catalyst were not changed by external dc fields up to 10 4 V/cm, however strong effects of the field on the adsorptive capacity of the catalyst were observed for T ≥ 200 °C. These results indicate that the rate-limiting step is not affected by the surface concentration of free electrons and also suggests that the overall reaction rate is not limited by adsorption-desorption steps.